The present invention relates to a DETECTION OF CROSS-LINKING IN PRE-CURE STAGE POLYMERIC MATERIALS BY MEASURING THEIR RESISTANCE.
1. Field of Invention
This invention relates to a process for determining the extent of cross-linking which has occurred in a polymeric material such as paint, dental resin, B-staged resin, etc. More particularly, the present invention pertains to the detection of extent of curing of such materials in their pre-cure stage.
2. Prior Art
Thermosetting resins form a class of very useful plastics which have been applied throughout the aerospace industry, construction industry, automotive manufacturing, medical applications, adhesives, and in virtually every area where permanent characteristics of weatherability, structural stiffness, strength and ease of manufacture through molding process provides an advantage over competing metals, ceramics and other compositions. Dental applications include filling and facia materials which are applied to the tooth in liquid form and then polymerized by UV radiation or other known techniques. Many paint compositions are a form of thermosetting resin whose application depends on having a uniform liquid state which can be readily applied by brush or air gun. Matched die, filament winding, transfer molding, lay up molding and pultrusion techniques for fabricating structural and component parts, housings, etc., depend on maintenance of a flowable condition which can wet fibers or quickly fill mold cavities in a liquid state.
These resin materials are typically manufactured in a low viscous liquid state wherein the polymer material has incurred minimal cross-linking prior to the curing stage. It is, of course, this cross-linking that solidifies the thermosetting composition into a permanent, rigid structure characterizing this group of plastics. The shelf life of such products is significant, because premature curing results in a permanent, irreversible condition which makes the material useless for further processing. Indeed, the extent of waste arising because of premature curing of thermosetting materials is substantial. In industries where partially cured materials must be discarded for safety reasons, the losses are even more significant. For example, the manufacture of high performance aircraft components from resins that have already partly cured could result in weakened structures that put life in jeopardy. Therefore, it is very likely that a substantial amount of good resin is discarded because of suspicion of excessive pre-cure.
Because most resins will inherently begin cross-linking upon manufacture and will continue such cross-linking until finally cured, measures are taken to reduce and control this process. The primary control measure is to maintain the resins at low temperatures to reduce reaction rates to a minimum. This low temperature environment needs to be maintained until the material is ready for final curing. Unfortunately, the resin material appearance does not always reflect the degree of curing which has occurred during this pre-cure stage. If variations in temperature occur during storage, their impact may be substantially unknown. Therefore, the extent of cure is often a risk factor that must be considered with the choice of any particular resin.
With paints and adhesives, viscosity provides a useful measure of acceptability of pre-cure. In general, their shelf life is determined by the time required for the material to set up or become too viscous to flow well. There are, however, no current tests to determine the actual state of cross-linking in paints and adhesives. Current practice is to examine the viscosity of the materials qualitatively as noted, or perform sample tests to determine the performance of these resins in a particular application.
With respect to polymers used in a matrix material for fiber reinforced composites, there are two distinct time periods during which cross linking takes place. The first period can be called the shelf life of the material and the second is the curing cycle. Typically, thermosetting resins for composites are stored at very low temperatures such as -0 degrees F. The curing cycle occurs when the resins are subjected to heat/radiation and/or pressure during molding processes. For example, elevated temperatures in the range of 200 to 400 degrees F., and occasionally as high as 700 degrees F., are common for curing these polymers and can enable the completion of cross linking in a short time interval.
Users of fiber reinforced thermosetting composites have created several mechanical tests to evaluate the state of cumulative cross linking in the storage and pre-cure stages. For example, tack and drape properties give an indication of the extent of cure. These tests are acknowledged to be highly subjective and unreliable, and are at best general qualitative indicators having little quantitative value.
A more specific application of thermosetting resins for composite materials is to impregnate a layer of fiber reinforcement with resin, and then store this "pre-preg" or "B-staged" material for later use. Obviously, this B-staged material will have a limited shelf life, depending upon the rate of continued cross linking, which is affected mainly by temperature. It is presently difficult, subjective, and consumptive of material to test the B-staged material for the extent of cross linking. If the B-staged material has reached a particular stage of cross linkage, it is no longer usable material and must be discarded on the basis of storage time, rather than on the actual amount of cross linking.
There is increasing interest in the composites industry to monitor, adjust and optimize the cure cycle of thermoset polymers. Accordingly, it is known to evaluate cross linking during actual cure using viscometers, infrared meters, and microdielectrometers. This period of evaluation is characterized by the resins being subjected to high temperatures used to fully complete the curing of the materials. The primary interest is to identify the gelation point and then to confirm final stage at which the curing process is complete, so that the final product can be removed without extending cure time and conditions beyond that which is necessary. This enables efficient use of expensive equipment and also insures that the manufactured part is not removed from the mold prior to complete cross linking.
The present inventors are unaware of any activity designed to determine the extent of cross linking in polymers prior to the actual curing process within a high temperature environment. Specifically, adhesives and paints represent a broad class of polymers which do not require devoted temperatures to cure to final stage. Because curing in such polymers is an ongoing process at a somewhat continuous rate, neither intermediate nor final cure status is generally measured. Where polymers are cured in two stages representing pre-cure and elevated final process, the only point of measurement of cross linking in polymers has occurred only during the elevated conditions, with little regard for cross linking during the pre-cure stage or shelf-life period.
This practice may have arisen in part from an assumption that the electrical response of any polymer at low temperatures applied during storage would not provide enough signal to show a measurable change as the pre-cure cross linking continues.
What is needed is an effective method for detecting the extent of cure in polymer materials during the shelf-life period, to enable more effective determination of whether specific batches or lots of polymer have exceeded safe limits in the pre-cure stage. Such procedures could provide quantitative determination of which resins must be discarded and which can be safely used, and yield substantial savings in cost and natural resources.